We develop a new class of two-stage mechanisms, which fully implement any social choice function under initial rationalizability in complete information environments. We show theoretically that our simultaneous report (SR) mechanisms are robust to small amounts of incomplete information about the state of nature. We also highlight the robustness of the mechanisms to a wide variety of reasoning processes and behavioral assumptions. We show experimentally that an SR mechanism performs well in inducing truth telling in both complete and incomplete information environments and that it can induce efficient investment in a two-sided holdup problem with ex ante investment.